This invention provides novel designs of laser resonators and laser wavelength tuning arrangements.
A typical example of existing methods for tuning the wavelength of a broad-band laser employs a laser resonator cavity consisting of a partially (or totally) reflecting mirror and one grating.
The grating is used in Littrow configuration at the oscillating wavelength, by which the ray at the oscillating wavelength is reflected back upon itself. For detailed description of a grating in Littrow configuration and other properties of a grating see, for example: Principles of Optics by Born and Wolf., Pergamon Press, New York (1959).
Such a resonator will provide regenerative feedback at the wavelength for which the grating angle is in Littrow with respect to the resonator's axis (determined by the direction normal to the resonator's fixed mirror). A broad-band amplifying medium placed within this resonator produces laser oscillation at the wavelength where the grating acts in Littrow, or in certain cases at a few closely spaced wavelengths near the peak of the grating's resolving band-width and determined by the various resonator modes which are generally spaced in frequency by c/(2L). Keeping the resonator mirror fixed and changing the grating angle changes the wavelength of the ray which will behave in Littrow as it propagates along the resonator axis. This then provides a means to wavelength tune a laser oscillator.
It is to be noted that such turning of the grating is in general used to provide coarse wavelength tuning over a wide region. Fine tuning of the laser is then obtained by keeping the grating angle fixed and changing the spacing between the mirror and the grating by a small amount. There are well known methods, such as piezoelectric tuning, where the latter can be achieved stably.
In other examples of existing methods of tuning, a laser cavity is employed in which a grating is fixed in non-Littrow position at least for some frequencies, and mirrors or Littrow gratings are placed to reflect rays of selected wavelengths diffracted by the grating, back upon themselves to the original grating, thence to the first mirror, see FIG. 2, Osgood, Sackett and Javan, Measurement of vibrational-vibrational exchange rates for excited vibrational levels (2.ltoreq.v.ltoreq.4) in hydrogen flouride gas, The Journal of Chemical Physics, Vol. 60, No. 4, Feb. 15, 1974. See also U.S. Pat. No. 3,928,817 and Friesem, Ganiel and Neumann, Simultaneous multiple wavelength operation of a tunable dye laser, Appl. Phys. Lett., Vol. 23, No. 5, Sept. 1, 1973.
Other arrangements for selection of wavelength or for simultaneous oscillation at multiple wavelengths exist, for example those shown in U.S. Pat. No. 3,872,407 and in Lotem and Lynch, Double-wavelength laser, Appl. Phys. Lett., Vol. 27, No. 6, Sept. 15, 1975. These and other prior art arrangements have disadvantages which the present invention overcomes.